Grating Spectra
| Filename | Description | Content | ICD | SDP Pipeline |
|---|---|---|---|---|
| acis_pha2.fits | ACIS L2 grating spectra | SPECTRUM | v1.4 | TG2 |
| hrc_pha2.fits | HRC L2 grating spectra | SPECTRUM | v1.4 | TG2 |
The creation process for ACIS and HRC gratings spectra is the same. A brief overview is given here; refer to the Analysis Guide for Chandra High Resolution Spectroscopy for complete details. Some the content presented here is from the Guide.
Description:
Transmission grating spectra for a Chandra observation are provided in the form of PHA files: 1-D counts histograms. (The name "PHA" is historical, referring to "Pulse Height Analyzer" or "Pulse Height Amplitude".) PHA are actually available in two formats, referred to as "Type I" and "Type II". Type I PHA are the traditional format of PHA, with sequential channels stored in sequential rows. Type II PHA files may contain many Type I PHA, with each Type I PHA stored as a row of the overall file. Thus, for any given row of a Type II PHA, the counts histogram is stored as an array. Since Chandra grating observations have at least two orders (for LETGS; 12 orders for HETGS), the Type II PHA is the standard method of storing grating observation data.
Among other things, the pipeline processing will find the grating arms using tgdetect, and assign each event to an order with tg_resolve_events. Finally tgextract is used to create the pha2.fits file.
To begin analysis on the pha2 file, see the Examining PHA2 Files thread.
Creator pipeline: TG2
tgextract - tgextract is the final step of a series of tools designed to process chandra grating data. See the Grating Spectroscopy threads page for info on reprocessing this data.
Useful links:
- Analysis Guide for Chandra High Resolution Spectroscopy - with acknowledgements for some of the content presented here.
- Grating Spectroscopy threads page
- TGCat - the Chandra Gratings Catalog and Archive provides reprocess data and pre-computed spectral products (i.e., spectra and response files).
SPECTRUM-specific columns:
| Column Name | Units | Description |
|---|---|---|
| SPEC_NUM | - | Spectrum Number (each row indicates a different PHA Type I) |
| TG_M | - | Diffraction order (m) |
| TG_PART | - | Spectral component (0=zero order, 1=HEG, 2=MEG, 3=LEG, 99=background) |
| TG_SRCID | - | Source ID, output by detect |
| X | pixel | X sky coord of source |
| Y | pixel | Y sky coord of source |
| CHANNEL[8192] | - | Vector of spectral bin numbers |
| COUNTS[8192] | count | Counts array (a spectrum) |
| STAT_ERR[8192] | count | Statistical uncertainty (error) on counts column |
| BACKGROUND_UP[8192] | count | Background count vector |
| BACKGROUND_DOWN[8192] | count | Background count vector |
| BIN_LO[8192] | angstrom | Bin boundry, left edge |
| BIN_HI[8192] | angstrom | Bin boundry, right edge |
The Type II PHA files will also have a second block called "REGION" attached. This block spatially defines the regions used to extract the source and background spectra for the observation. For example, an HETG/ACIS observation has a source, upper background, and lower background defined for each of the six orders of the HEG and MEG spectra. This creates a total of 36 regions.
REGION-specific columns:
| Column Name | Units | Description |
|---|---|---|
| SPEC_NUM | - | Spectrum Number (each row indicates a different PHA Type I) |
| ROWID | - | Source or a background region? |
| SHAPE | - | Shape of region |
| TG_LAM | angstrom | Dispersion coordinate vector for SHAPE |
| TG_D | degrees | Cross-dispersion coordinate vector for SHAPE |
| R[2] | (angstrom , degrees) | Radius vector for SHAPE |
| ROTANG | degrees | Rotation angle for SHAPE |
| TG_PART | - | Grating part index (HEG=1, MEG=2, LEG=3) |
| TG_SRCID | - | Source identification number |
| TG_M | - | Diffraction order |